There is a focus detection device known in the related art that includes a micro-lens array disposed at a predetermined focal plane of an image forming optical system and a plurality of light receiving elements disposed in a linear pattern in correspondence to each micro-lens, creates a pair of signal value strings made up with signal values output from the light receiving elements disposed below the micro-lenses, which correspond to a pair of areas of the pupil of the image forming optical system, and detects the focusing condition at the image forming optical system by detecting the shift amount manifested by the pair of signal value strings (see patent literature 1).
Patent Literature 1: Japanese Laid Open Patent Publication No. 2007-11314